The acrylic acid synthesis process employed on a large industrial scale implements a reaction for catalytic oxidation of propylene in the presence of oxygen.
This reaction is generally carried out in the gas phase, and usually in two steps: the first step carries out the substantially quantitative oxidation of the propylene in an acrolein-rich mixture, and then, during the second step, the selective oxidation of the acrolein to acrylic acid is carried out.
The gas mixture resulting from the second step consists, other than of the acrylic acid:    of the impurities resulting from the first reaction step which have not reacted (propylene, propane);    of light compounds which are non-condensable under the temperature and pressure conditions normally used, and which are not converted in the first step or are formed in the second step: nitrogen, unconverted oxygen, carbon monoxide and dioxide formed in a small amount by final oxidation or going around in circles, by recycling, in the process;    of condensable light compounds which are not converted in the first step or are formed in the second step: water, unconverted acrolein, light aldehydes, such as formaldehyde and acetaldehyde, formic acid, acetic acid or propionic acid;    of heavy compounds: furfuraldehyde, benzaldehyde, maleic acid and anhydride, benzoic acid, 2-butenoic acid, phenol, protoanemonin.
The complexity of the gas mixture obtained in this process makes it necessary to carry out a set of operations in order to recover the acrylic acid contained in this gas effluent and to convert it into a grade of acrylic acid compatible with its final use, for example the synthesis of acrylic esters or the production of polymers of acrylic acid and/or of acrylic esters.
A new acrylic acid recovery/purification technique has recently emerged, involving a reduced number of purification steps and not requiring any external organic solvent.
Patent EP 2 066 613, based on this “solvent-free” technique, describes a process for recovering acrylic acid without using external water or azeotropic solvent and using only two columns for purifying the cooled gas reaction mixture: a) a dehydration column, b) and a finishing column (or purification column) fed with a part of the stream from the bottom of the dehydration column.
According to this process, the cooled gas reaction stream is subjected to dehydration in a first column. The gas stream distilled at the top of the column is sent to a condenser, in which the light compounds are partly condensed and sent back to the dehydration column in reflux form in order to absorb the acrylic acid, the gas effluent being at least partly sent back to the reaction and the remainder being incinerated.
The stream from the bottom of the dehydration column feeds a second column which makes it possible to separate, by drawing off from the side, in liquid or vapour form, a stream of purified acrylic acid corresponding to a technical grade. The technical-grade acrylic acid obtained generally has a purity greater than 98.5% by weight and contains less than 0.5% by weight of water.
In this finishing column, the top distillate comprising water and light by-products is condensed and then recycled to the bottom of the first column, and a stream comprising acrylic acid enriched with heavy by-products is eliminated at the bottom so as to optionally be used for the production of acrylic esters.
In this process, a part of the streams (from the bottom of the dehydration column or from the top of the finishing column) is advantageously sent back to the heating/reboiler devices of the dehydration column and/or used to cool the gas reaction mixture, thereby making it possible to optimize the energy requirements of the process.
Despite the advantages provided by the purification process described in document EP 2 066 613, there still remain drawbacks associated with its implementation, in particular in terms of the possible loss of acrylic acid during the various steps.
In particular, acrylic acid can be entrained at the top of the dehydration column. Depending on the liquid/vapour equilibrium at the operating temperature of the condenser placed at the top, the gas effluent at the outlet may contain acrylic acid in a not insignificant amount. Acrylic acid is directly lost in the part of the gas effluent that is incinerated.
There thus remains a need to reduce the acrylic acid losses in a solvent-free recovery/purification process based on the use of a dehydration column and of a finishing column, and in particular to reduce the acrylic acid losses at the top of the dehydration column.
The inventors have now discovered that the acrylic acid loss can be reduced by controlling the content of water introduced into this process.
The main source of water comes from the crude reaction mixture to be treated, since it contains the water formed during the reaction for catalytic oxidation of the propylene to acrylic acid. The water is the main “light” impurity that is distilled at the top of the dehydration column, and its elimination during the purification process conditions the quality that is sought for the purified acrylic acid (water content <0.5% by weight).
It is necessary to eliminate the water in an optimal manner at the top of the dehydration column while at the same time minimizing the acrylic acid loss. It is possible, for example, to reduce the temperature of the condenser placed at the top, so as to modify the liquid/vapour equilibrium and to prevent entrainment of acrylic acid in the gas effluent. However, this is possible only within a certain limit that depends on the water content introduced into the dehydration column.
In addition to the water inherent in the acrylic acid synthesis process, water is introduced by other routes, such as, for example, the moisture content present in the air stream introduced into the reaction in order to carry out the oxidation of the propylene. Moreover, aqueous solutions of polymerization inhibitors are introduced into the columns and/or condensers in order to limit the polymerization reactions, and the water thus introduced can go around in circles in the process by recycling and/or reflux.
It has been discovered that, by reducing these supplementary sources of water, the elimination of the water at the top of the dehydration column can be optimized, while at the same time minimizing the acrylic acid losses.
According to the invention, it is proposed to condense the water present in the air stream feeding the reaction, and/or the water present in the gas effluent from the dehydration column which is recycled to the reaction.
Moreover, it has become apparent to the inventors that this invention can be applied to the acrylic acid produced from sources other than propylene, to methacrylic acid, and also to these acids derived from renewable raw materials, which are capable of posing the same purification problems associated with the presence of water.